Tuesday, October 17 2006
The premature chill culminated today with a long dreary rain, the gentle persistent kind usually indicative of a warm front. At this time of year, warm fronts are usually welcome.
The recent chill had us on the verge of starting up the boiler for the season, but before we do that I wanted to make a few adjustments to the way it operates. Half the household hydronic zones (including the one that heats the water when the sun isn't up to the task) are part of a system controlled by a L8124A Honeywell triple aquastat. This aquastat decides whether or not to fire the boiler and whether or not to fire the zone circulator pump based on the answer to three questions: Is a zone calling for heat? Is the water temperature in the boiler below the "must cut on threshold"? Is the water temperature in the boiler above the "oh shit, we're about to blow up!" threshold? The two thresholds are set by little knobs that can be adjusted, and there's also a knob for "differential setting" but the documentation never really explained what that meant. My problem with this system was that any time a thermostat in any of the three zones controlled by this mechanism called for heat, it wasn't enough for the valves to open and the circulator pump to cut on. No, the boiler had to fire as well. I wanted these zones to behave more like the other zones and just circulate water when they needed heat, leaving the boiler to decide, based only on the temperature of its water, whether or not to fire. Then, by setting the lowest and highest boiler range thresholds far apart, I could increase the fire time of the boiler and raise its efficiency. (According to my new energy-conservation-obsessed friend Wolfgang, it is possible to increase a boiler's efficiency by 40 percent if one does this correctly.)
I was able to find complete documentation for the Honeywell aquastat, though the schematics didn't make any sense. I've seen a lot of schematics in my day, and this one was completely alien. Where was the symbol for the relay that was obviously inside? And, in the schematic, there were lots of symbols that looked like capacitors (two wires coming together but not quite touching, terminated in serifs), each labeled "1K2" or "1K3," though there was no reason for there to be any capacitors in this system at all (and in any case, capacitor values are never given in units prefixed with "kilo"). There was also what appeared to be a resistor labeled "1K." None of these symbols were mentioned in the documentation at all. I did some internet research and turned up nothing. Happily, though, in the future the internet will have an answer and it will be on this page.
Suddenly I realized what the symbols meant. All the symbols labeled with "1K" were parts of a relay. The things that looked like capacitors were actually relay contacts and the thing that looked like a resistor was actually the activation coil for that relay (why else would it be attached to the thermostat wires?). Once I had that knowledge in my head, it was possible to rewire the aquastat to behave the way I'd set out to have it behave. The boiler is still off and I haven't tested it to see if it will work as expected, but the theory behind it all is just too elegant for it not to work.
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